This application claims the priority benefit of Taiwan application serial no. 89106779, filed Apr. 12, 2000.
1. Field of the Invention
The invention relates in general to a method of measuring the thickness of a cell gap of a reflective type liquid crystal display (LCD). More particularly, the invention relates to a method of measuring a thickness of a cell gap of a reflective type mixed-mode twisted nematic (MTN) liquid crystal display.
2. Description of the Related Art
In the recent years, the liquid crystal display, being supported with the development of relative electronic devices, becomes very widely applied with versatile functions. Consequently, the fabrication of liquid crystal displays becomes more complex. Generally speaking, the liquid crystal display can be categorized into reflective type liquid crystal display, transmissive type liquid crystal display and transflective type liquid crystal display.
In the liquid crystal display, the cell gap is a very important factor to control the characteristics such as brightness, contrast or color. In the current technique for fabricating the liquid crystal display, only the transmissive type liquid crystal display has a measuring method to precisely measure the thickness of the cell gap. This method cannot be applied to the reflective type or transflective type liquid crystal display. If one applies this method to measure the cell gap of the reflective type liquid crystal display, the surface reflection may seriously interfere the correctness of the measurement of the cell gap.
The invention provides a method of measuring a cell gap of a reflective type liquid crystal display. In addition to precisely measure the thickness of the cell gap, the methods can also eliminate effect induced from the surface reflection of the reflective type liquid crystal display.
A relationship between the maximum value xcex2max of a beta angle xcex2 and a thickness of the cell gap is derived to calculate the thickness of the cell gap in this method.
In the method provided by the invention, an optical system is provided. The optical system comprises a light source, a rotating table, an input polarizer, a beam splitter, a output polarizer and a photodiode. The light source includes a He/Ne laser to produce a light beam incident to the input polarizer. A reflective type liquid crystal display device, for example, a reflective type mixed-mode twisted nematic (MTN) is disposed on the rotating table. The reflective type liquid crystall display device comprises a front liquid crystal director and a rear liquid crystal director. The front liquid crystal director indicates the liquid crystal director at the surface of the reflective type liquid crystal display near the beam splitter. The input polarizer is used to receive and polarize the light beam incident from the light source. The beam splitter receives the light coming from the input polarizer. Two light beams are obtained and output by the beam splitter. One light beam is incident back to the reflective type liquid crystal display and reflected thereby. The output polarizer is used to receive the light beam reflected by the liquid crystal display. Along the optical path, the input polarizer and the output polarizer are located at two sides of the reflective type liquid crystal display. The transmissive axis of the input polarizer is perpendicular to the transmissive axis of the output polarizer. The photodiode is used to receive the light beam from the output polarizer to convert the light beam into an electric current signal.
A beta angle xcex2 is defined as the angle between the input polarizer and the front liquid crystal director. A formula as follow is provided:             R      ⊥        =                            (                      Γ            ⁢                          xe2x80x83                        ⁢                                          sin                ⁢                                  xe2x80x83                                ⁢                X                            X                                )                2            ⁢              xe2x80x83            ⁢                        (                                    sin              ⁢                              xe2x80x83                            ⁢              2              ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              X                        -                                          φ                X                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              2              ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢              sin              ⁢                              xe2x80x83                            ⁢              X                                )                2              ,
wherein
xcex93=2xcfx80dxcex94n/xcex,       X    =                            φ          2                +                              (                          Γ              /              2                        )                    2                      ,
d is the thickness of the cell gap, xcex is about 632.8 nm, xcex94n is the birefringence (about 0.064), xcfx86 is the twisted angle of the liquid crystal, that is, the angle between the front and back liquid crystal directors, about 80xc2x0 to 90xc2x0.
The reflectivity Rxe2x8axa5 is differentiated by the beta angle xcex2 to obtain a formula:                     ⅆ                  R          ⊥                            ⅆ        β              =                  0        ⇒                  β          max                    =                                    tan                          -              1                                ⁡                      (                          -                              X                                  φ                  ⁢                                      xe2x80x83                                    ⁢                  t                  ⁢                                      xe2x80x83                                    ⁢                  tan                  ⁢                                      xe2x80x83                                    ⁢                  X                                                      )                          2              ,
wherein xcex2max is a function of d, xcex94, xcfx86, and xcex.
According to the second formula, with constant xcex94, xcfx86, xcex, xcex2max is only the function of the thickness of cell gap d. The rotating table is then rotated to measure the maximum value of the beta angle xcex2max, so that the thickness of the cell gap d can be derived.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.